The effect of pentobarbitone on variable-interval performance: analysis in terms of Herrnstein's equation.

Six rats were exposed to a series of variable-interval schedules of reinforcement, each schedule specifying a different reinforcement frequency. The effects of two doses of pentobarbitone (24.16 and 40.27 mumol/kg) upon performance maintained under each schedule were examined. The data were analyzed in terms of the goodness of fit of Herrnstein's (1970) equation. Under control conditions (no injection or injection of vehicle alone) response rate was an increasing negatively accelerated function of reinforcement frequency, the data obtained from each rat conforming closely to Herrnstein's equation. In the presence of pentobarbitone there was a reduction in the value of Rmax (the theoretical maximum response rate) in all six rats. The effect of pentobarbitone upon the value of KH (the reinforcement frequency needed to obtain the half-maximal response rate) differed between animals, the value being increased in some rats and decreased in others. This was shown to reflect a systematic relationship between the baseline value of KH and the change in KH produced by pentobarbitone, high baseline values being associated with reductions, and low baseline values with elevations, in the presence of the drug. The data were also analyzed using conventional rate-dependency analysis. In some animals there was a tendency for lower response rates to be suppressed to a greater degree than higher rates, while the reverse was true in other animals. In general, rate-dependency functions provided poor descriptions of the data obtained from individual subjects.